This change adds constant folding support for unary common built-ins:
abs, sign, floor, trunc, round, roundEven, ceil and fract.

BUG=angleproject:913
TEST= dEQP tests
dEQP-GLES3.functional.shaders.constant_expressions.builtin_functions.common*
(80 out of 210 tests started passing with this change)

Change-Id: I06800ed0e03764c0f7aab6bcd45c4f122de5a3c1
Reviewed-on: https://chromium-review.googlesource.com/265394Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>